The present invention relates to a mobile satellite communication system which affords voice channels and digital data channels to a number of mobile stations that are distributed over a wide area on the earth.
In parallel with the improvement in the capability of a communication satellite, a satellite communication system that allows an earth station to directly connect itself to a communication satellite with miniature antenna has been extensively developed. This kind of communication system allows the small-size configuration of an earth station, and has other various advantages. A prior art satellite communication system of the type described, however, has a problem of extremely limited output effective isotropic radiation power (EIRP) and reception gain-to-noise temperature (GT) ratio attainable with the earth station, and the problem that the frequency resources allocated to mobile station communications are scarce. The EIRP and reception GT ratio problem may be coped with by adopting spot beams which are effective in increasing the antenna gain of a satellite. Additionally, the frequency resource problem may be eliminated by adopting spot beams which promote frequency re-use and dividing a frequency band used into a number of narrow band channels. For example, NASA's MSAT (Mobil Satellite Experiment) program contemplates a method that uses eighty-seven spot beams and a method that transmits voice and data by using a channel band of 5 kilohertz. Omninet Corp of U.S.A is planning to transmit voice in a 5-kilohertz channel band by SSB-AM (Single Sideband Amplitude Modulation).
All the mobile satellite communications proposed above apply an FDM (Frequency Division Multiplex) system to both of an uplink and a downlink. Although the FDM system may be successful in simplifying the construction of a transmitter and that of a receiver of a ground station, it gives rise to cross modulation noise because it causes an onboard transponder to apply common amplification to FDM signals. To insure acceptable quality despite cross modulation, it is necessary that the operation points of an amplifier be selected with sufficient back-offs with respect to the saturation point. This brings about EIRP losses corresponding to the back-offs. One possible approach to solve this problem may be transforming FDM signals into TDM (Time Division Multiplex) signals on a satellite. For example, an onbaord FDM-TDM conversion method which relies on chirp Z transform has been proposed by Nippon Telephone and Telgraph Corp. However, the chirp Z transport principle makes the operation extremely difficult and, apparently, cannot eliminate crosstalk between various channels, or interchannel interference, in the case of transmission of analog signals.
Further, none of the mobile satellite communication systems presently under study teaches how to interconnect signals between beams on a satellite. For example, both the NASA's MSAT program and the Omninet's program cause a ground network to effect signal connection between beams via base stations that are associated with respective spot beams. Hence, the prior art systems fail to make the best use of wide coverage which is particular to satellite communications.